An NOx selective reduction catalyst that selectively reduces NOx in the exhaust gas using ammonia as a reducing agent is provided in an exhaust passage of the internal combustion engine in some cases. In such cases, urea is supplied to the NOx selective reduction catalyst by urea supply means. Urea supplied to the NOx selective reduction catalyst is once adsorbed by the NOx selective reduction catalyst, and the adsorbed urea is hydrolyzed to produce ammonia. The ammonia serves as a reducing agent to reduce NOx in the exhaust gas.
In this case, ammonia that has not been consumed in reduction of NOx may be discharged from the NOx selective reduction catalyst. In view of this, an ammonia oxidation catalyst having the function of oxidizing ammonia is provided in the exhaust passage downstream of the NOx selective reduction catalyst in some cases.
In such cases, if ammonia is oxidized in the ammonia oxidation catalyst in a state in which the temperature of the ammonia oxidation catalyst is sufficiently high, NOx is produced. On the other hand, if ammonia is oxidized in the ammonia oxidation catalyst in a state in which the temperature of the ammonia catalyst is relatively low, N2O may be produced. If this is the case, the quantity of ammonia and NOx discharged downstream of the ammonia oxidation catalyst decreases, and N2O is discharged. However, since N2O is a gas that contributes to a green house effect, it is desirable that emission of N2O to the atmosphere be reduced.
Japanese Patent Application Laid-Open No. 2000-230414 discloses an arrangement in which an NOx adsorbent and an NOx selective reduction catalyst are provided in series in order from the upstream in an exhaust passage of an internal combustion engine. In the arrangement disclosed in Japanese Patent Application Laid-Open No. 2000-230414, NOx having slipped through the NOx adsorbent is reduced in the NOx selective reduction catalyst into N2 or N2O.